What is mediumband?

Fig. 1: Mediumband region in the delay spread-symbol period plane [1]

Importantly, mediumband does not refer to any particular band of frequencies in the electromagnetic (EM) spectrum, and also is not a modulation scheme or a coding scheme or a transmitter precoding scheme or a receiver combining scheme or a detection algorithm, or an anything of that sort.

It, in essence, is a class of wireless channels or waveforms. For those who are not familiar with the concept of wireless channels/waveforms, mediumband could be regarded as a set of constraints defined in terms of two important parameters in wireless communication: delay spread and symbol period as shown in Fig. 1 [1].

A mediumband wireless channel means a communication scenario where the delay spread is significantly larger in comparison to the symbol period. Consequently, wireless systems operating in this class, or region, experience increased level of inter-symbol-interference (ISI) reducing signal-to-interference-ratio (SIR). On the other hand, the desired fading factor in mediumband channels exhibits some thing called deep fading avoidance [2].

The performance of wireless communication in the mediumband is dictated not just by the average SIR, but also the net effect of SIR and the effect of deep fading avoidance, which can successfully counter the adverse effect of the increased level of ISI [2]. What is this effect of deep fading avoidance?

As shown in Fig. 2, the effect of deep fading avoidance in mediumband channels is such that the probability density function (PDF) of the desired fading factor that arises in mediumband wireless systems exhibits a dip (or hole) at zero signifying the fact that the probability of the desired fading factor being in deep fade is low.

Fig. 2:  Depictions of the resulting PDFs (only a single dimension) of the desired fading factor in several methods known to reduce deep fading [2].

In modern wireless communication systems like 4G, 5G, WiFi, the information bearing signal is a broadband waveform. Using IFFT and FFT blocks at the transmitter and receiver respectively, the broadband waveforms are traditionally designed as a set of parallel narrowband waveforms. However, narrowband waveforms have many drawbacks. Because, their signalling rate is low and are exposed to excessive deep fading. The research have shown that it is more favourable to make broadband waveforms, not as a set of parallel narrowband waveforms, but as a set of parallel mediumband waveforms.

REFERENCES

1. ​​D. A. Basnayaka, "Introduction to mediumband wireless communication," in IEEE Open Journal of the Communications Society, vol. 4, pp. 1247-1262, May. 2023. (Open Access)

   
   

2. D. A. Basnayaka, "Communicating in the mediumband: What it is and why it matters," IEEE Communication Magazine, vol. 62, no. 11, Nov. 2024. (Open Access)